Description: The goal of this project is to enhance our understanding of the effects of soil heterogeneity and rate-limited sorption/desorption on biodegradation and in-situ bioremediation of organic contaminants in subsurface systems. The specific objectives that will be addressed in this proposed research will be to: 1) investigate the influence of physical heterogeneity ("preferential flow" and micro- pore/macro-pore mass transfer) on location and rate of biodegradation during transport; 2) investigate the effect of contact time (between contaminant and soil) and rate-limited desorption on bioavailability; 3) develop mathematical models for simulating biodegradation, rate- limited sorption/desorption diffusive mass transfer and transport in the heterogenous porous media. A novel approach is proposed to characterize the physical (pore-sized distribution, impact of heterogeneity on fluid transport), chemical (rate in extent of sorption), and microbiological (location, concentration, and activity of bacteria) properties of soil contained in the columns used to measure contaminant transport. This approach combines flow and transport characterization techniques (water- retention function, tracer transport, dye staining) with sorption measurements and advanced techniques for characterizing the nature and quantity of bacteria population (radio-label cells, gene probe). This approach will allow them to correlate the impact of soil heterogeneity on flow and transport to the impact of soil heterogeneity on the rate of sorption/desorption and on the location and rate of biodegradation.